Fix Windows IMMNotificationClient having non-virtual destructor warning

[madmiraal]

IMMNotificationClient is a Microsoft COM Interface. It is a pure virtual class that does not have a virtual destructor.

Windows Audio Session API (WASAPI) uses IMMNotificationClient to provide notifications when an audio endpoint device is added or removed, when the state or properties of an endpoint device change, or when there is a change in the default role assigned to an endpoint device.[1]

To receive notifications when the default audio endpoint device for a particular device role has changed, Rebel Engine passes a reference to an instance of CMMNotificationClient (an implementation of IMMNotificationClient) when registering the endpoint notification callback. However, CMMNotificationClient is not implemented as a COM Object. It simply extends the C++ virtual class. This results in the CMMNotificationClient declaration warning:

base class 'struct IMMNotificationClient' has accessible non-virtual destructor

This warning alerts against the possible undefined behaviour when deleting a pointer to the base class IMMNotificationClient, which would not call the destructor of CMMNotificationClient. This issue is avoided in COM programming by only creating COM Objects using CoCreateInstance and making the COM Object responsible for reference counting and deleting itself.[2][3]

To ignore the warning in Rebel Engine, we need to independently ensure that CMMNotificationClient is not deleted via a pointer to IMMNotificationClient. In Rebel Engine, CMMNotificationClient is a static variable; so it cannot be deleted. It is passed by reference (i.e. not a pointer) to RegisterEndpointNotificationCallback().

The standard implementation of IUnknown COM Interface that all COM Objects inherit, uses the Release() method to decrement the reference counter and delete itself when the reference counter reaches zero. Since CMMNotificationClient is not implemented as a COM Object, this PR also ensures that CMMNotificationClient is never returned as an interface, provides dummy implementations of the other IUnknown Interface methods, and removes the unused reference counter variable.

Finally, it also removes the unused _pEnumerator variable.

Fixes godotengine/godot#35194

[1] https://learn.microsoft.com/en-us/windows/win32/api/mmdeviceapi/nn-mmdeviceapi-immnotificationclient
[2] https://learn.microsoft.com/en-us/windows/win32/learnwin32/creating-an-object-in-com
[3] https://learn.microsoft.com/en-us/windows/win32/com/managing-object-lifetimes-through-reference-counting

[DanielaOrtner]

Thank you for the good work!